Sputtering is a well established technology in the fabrication of silicon integrated circuits, in which a metal target is sputtered to deposit target material onto the silicon wafer. Sputtering has also been applied to other uses, such as window coatings. In recent years, sputtering has also been applied for similar purposes as for silicon integrated circuits in the fabrication of flat panel displays, such as flat computer displays and large flat televisions and the like. Various types of flat panel displays may be fabricated typically including thin film transistors (TFTs) formed on large thin insulating rectangular substrates, often called panels, and including liquid crystal displays (LCDs), plasma displays, field emitters, and organic light emitting diodes (OLEDs).
A conventional flat panel sputter reactor 10 is schematically illustrated in the cross-sectional view of FIG. 1. Demaray et al. (hereafter Demaray) disclose more details of such a reactor in U.S. Pat. No. 5,565,071, incorporated herein by reference. A pedestal 12 within a main vacuum chamber 14 supports a rectangular panel 16 to be sputter coated in opposition to a generally rectangular target tile 18 bonded to a backing plate 20 sealed to but electrically isolated from the main chamber 14 by an isolator 22. The panel 16 may be composed of a glass, a polymeric material, or other material. The target material is most typically a metal such as aluminum, molybdenum, or indium tin oxide (ITO) although other metals may be freely substituted depending on the type of layer desired to be formed on the panel 16. Larger targets may require the bonding of multiple target tiles to the backing plate in one- or two-dimensional arrays. An unillustrated vacuum pump system pumps the interior of the main chamber 14 to a base pressure of 10−6 to 10−7 Torr or below. A gas source 24 supplies a sputter working gas such as argon into the chamber 14 through a mass flow controller 26 and the main chamber pressure is kept typically at no more than a few milliTorr during sputtering. A DC power supply 28 applies a negative DC bias of several hundred volts to the target 18 in opposition to the grounded pedestal 12 and unillustrated chamber shield to cause the argon to be excited into a plasma. The positively charged argon ions are attracted and accelerated by the negatively biased target 18 with sufficient energy to sputter atoms of the target material from it. Some of the sputtered material strikes the panel 16 and coat it with a thin layer of the target material. Optionally, a reactive gas such as nitrogen, may be additionally admitted to the chamber to cause the sputtered metal to react with it and form a metal compound such as a metal nitride on the panel surface.
Sputtering is greatly enhanced if a magnetron 30 having opposed magnetic poles is placed in back of the backing plate 20 to project a magnetic field B into the main chamber in front of the target 18. The magnetic field traps electrons and thus increases the density of the plasma adjacent the target 18, greatly increasing the sputtering rate. To achieve uniform erosion of the target 18 and uniform deposition on the panel 16, the magnetron 30 is scanned in a one- or two-dimensional pattern across the back of the backing plate 20. The form of the magnetron 30 may be much more complex than that illustrated.
Almost all panel fabrication equipment is distinguished by its large size. The original generation was based on panels having lateral dimensions of the order of 500 mm. Various economic and product factors have prompted successive generations of flat panel fabrication equipment of ever increasing sizes. The next generation is being developed to sputter deposit on panels having sides of greater than 2 m. This large size has introduced several problems not experienced in wafer fabrication equipment limited to sizes of about 300 mm in the most recent equipment.
The target 18 and more particularly its backing plate 20 must be relatively thin so that the magnetron 30 can project a substantial magnetic field through it. However, absent other means, the backing plate 20 needs to stand off a considerable force (differential pressure times the area) between its back and the high vacuum of the main chamber 14 and further the backing plate 20 should not significantly bow under these pressure differentials. To provide such large thin targets, Demaray proposed placing the magnetron 30 inside a magnetron chamber 32 sealed to the back of the backing plate 20 and pumped to a relatively low pressure in the sub-Torr range, the limit of a mechanical vacuum pump. Such back pumping reduces the force exerted on the backing plate 20 by a factor of about a thousand.
Such a structure contrasts with a conventional wafer sputter reactor in which a corresponding chamber at the back of the target backing plate 20 is filled with chilling water to cool the target during sputtering. Demaray, instead, recirculates cooling liquid from a chiller 34 through cooling channels formed within the backing plate 32. As shown in the cross-sectional view of FIG. 2, a substantially rectangular conventional target 40 includes a backing plate 42 formed of top and bottom plates 44, 46. Cooling channels 48 of generally rectangular cross section are machined into the surface of the top plate 44 to extend generally between the two sides of the backing plate 42 although larger horizontal distribution manifolds may be formed nearer the two sides to connect the cooling channels 48 to a common cooling liquid inlet and a common cooling liquid outlet. The bottom plate 46 is then bonded to the top plate 44 to enclose and seal the cooling channels 48 and manifolds. A target tile 50 is then bonded to the backing plate 42. In the past, indium bonding was most often used but conductive polymeric adhesive bonding is gaining favor.
The bonding of the two plates 46, 48 of the backing plate 42 has presented technical challenges, particularly at the larger panel sizes. It is desired to reuse the backing plate 42 when sputtering has effectively eroded through the target tile 50. That is, it is desired to remove the old target tile 50 and replace it with a new one. The backing plate 42 needs to be rugged to survive refurbishment when the used target tile is delaminated from the backing plate and a new target tile is laminated. Targets and their backing plates have become increasingly expensive for the larger sizes of panels. Thus, their cost should be reduced while their ruggedness should be maintained and preferably increased. The two plates 44, 46 can be welded together, but welding tends to deform thin plates. The two plates 44, 46 can be screwed together with a sealant placed in the interface. However, the number of screws required for a 2.5 m×2.5 m target becomes very large. Indium bonding can be used, but its ruggedness is questionable. Autoclaving has been suggested, but this is a complex and expensive process.
The larger target sizes have also presented a challenge in uniformly cooling a larger area without unduly increasing the thickness of the target assembly.